What is Significance of Plasticity Index of Soil?

Plasticity index

This is equal to difference between liquid limit and plastic limit of soil.

Plasticity index is also denoted by PI. This is the range of the water content within which soil achieves its plastic state. This is a measure of plasticity of soil of our interest.

If

Wl =liquid limit

Wp=plastic limt

Ip=plasticity index

This parameter cannot be negative if plastic limit, in some exceptions, is larger than liquid limit, ignoring negative value, it is considered to be zero.

A soil is considered non-plastic when liquid limit or plastic limit cannot be determined and denoted by NP. Clay soils having high plasticity index with high liquid limit, is called highly plastic clays; sometimes called fat clays. Those have low values called lean clays.

Both liquid limit and plastic limit are determined from remolded soils. Thus natural structure of soil is disturbed. If actual behaviors have to be determined, we should test soil in undisturbed condition.

As plasticity index is a product of these limits, it will not provide us complete idea about soils. A rough information about the characteristics of soil at best can be determined.

Plasticity index and liquid limit are considered important in co-relating engineering properties, as they are influenced by identical factors like

• For a same plasticity index, when liquid limit increases permeability and compressibility are found to be increased whereas toughness and dry strength is decreased.

• For a same liquid limit of two samples, when plasticity index is increased, permeability decreases whereas toughness and dry strength are increased. But compressibility is found almost unchanged.

• Soils having high plasticity index are considered clay and those having lower value are considered silt. In case of zero value, soil are considered to have little/no clay or silt and called non-plastic soil.
• A lower plasticity index of two soil is indicative to have high organic matter in soil.

We have all know more or less about plasticity chart this is nothing but a plot with plasticity index against liquid limit. Plasticity chart is extensively used in classifying fine grained soil.

when i have a Pl of 10 and a Ll of 15, the Pi will be 5 so i read above that when the Pi is low the soil has a high organic matter. my question is about what do we say the PL is either low or high in order to classify the soil

when i have a Pl of 10 and a Ll of 15, the Pi will be 5 so i read above that when the Pi is low the soil has a high organic matter. my question is about what do we say the PL is either low or high in order to classify the soil